Explanation: Many bright nebulae and star clusters in planet Earth's sky are associated with the name of astronomer Charles Messier, from his famous 18th century catalog. His name is also given to these two large and remarkable craters on the Moon. Standouts in the dark, smooth lunar Sea of Fertility or Mare Fecunditatis, Messier (left) and Messier A have dimensions of 15 by 8 and 16 by 11 kilometers respectively. Their elongated shapes are explained by a left-to-right moving, extremely shallow-angle trajectory followed by an impactor that gouged out the craters. The shallow impact also resulted in two bright rays of material extending along the surface to the right, beyond the picture. Intended to be viewed with red/blue glasses (red for the left eye), this striking stereo picture of the crater pair was recently created from high resolution scans of two images (AS11-42-6304, AS11-42-6305) taken during the Apollo 11 mission to the moon.

Given our knowledge of the gravity of the moon and some knowledge of its composition, can we arrive at a fair estimate of the size of the bodies that caused these impacts? Speed and composition of the objects would remain as unknowns. The Arizona Crater, the last I heard, was caused by an object the size of a railroad boxcar.

<<In an instantly classic study, Don Gault of NASA's Ames Research Center and John Wedekind of Caltech reported in 1978 that the shape of an impact crater and its ejecta pattern depend on the angle at which a projectile strikes a target. Using the Ames Vertical Gun Ballistic Range, they shot millimeter-size spheres and cylinders into targets of granite, quartz sand, and volcanic pumice powder at velocities up to 7 kilometers per second. ... not much changes until the impact angle is less than 45° (measured from horizontal). But at shallower angles the crater becomes increasingly elongated in the direction of motion, and portions of the projectile ricochet and gouge out a series of small pits downrange from the main crater. As the impact angle changes, the ejecta and rays undergo even more pronounced changes than the craters do. When the impact angle is less than 15°, the ejecta pattern becomes elongated in the downrange direction and a "forbidden zone," where no ejecta appears, develops in the uprange direction. For grazing impacts of just a few degrees, the rays go sideways only, producing a butterfly-wing pattern.

The really remarkable fact about Messier and Messier A is that the scientists Gault and Wedekind were able to beautifully mimic every one of the pair's weird features in laboratory impact experiments. A grazing impact (1° to 5°) of a projectile coming from the east excavated Messier (explaining its elongated shape and classic butterfly-wing ejecta pattern) and another part of the projectile ricocheted downrange to form Messier A and its long rays. Bigger craters formed obliquely too — look closely at Proclus, Kepler, Tycho, and even Mare Crisium.>>

NCTom wrote:Given our knowledge of the gravity of the moon and some knowledge of its composition, can we arrive at a fair estimate of the size of the bodies that caused these impacts? Speed and composition of the objects would remain as unknowns. The Arizona Crater, the last I heard, was caused by an object the size of a railroad boxcar.

A glancing collision constrains things better, allowing for better estimates. It's a more complex problem with non-glancing impacts like the Arizona Crater.

NCTom wrote:
Given our knowledge of the gravity of the moon and some knowledge of its composition, can we arrive at a fair estimate of the size of the bodies that caused these impacts? Speed and composition of the objects would remain as unknowns. The Arizona Crater, the last I heard, was caused by an object the size of a railroad boxcar.

Meteor Crater in Arizona was caused by an object with the volume of a thousand railroad boxcars.

Messier Crater was caused by an object the with a volume a thousand times larger.

(The main effect of the Earth's gravity & atmosphere is to minimize meteor craters caused by glancing blows.)

https://en.wikipedia.org/wiki/Meteor_Crater wrote:
<<Meteor Crater is about 1.2 km in diameter, some 170 m deep, and is surrounded by a rim that rises 45 m above the surrounding plains. The object that excavated the crater was a nickel-iron meteorite about 50 meters (160 feet) across. The speed of the impact has been a subject of some debate. Modeling initially suggested that the meteorite struck at up to 20 kilometers per second (12 miles per second) but more recent research suggests the impact was substantially slower, at 12.8 kilometers per second (8.0 miles per second). It is believed that about half of the impactor's bulk was vaporized during its descent. Impact energy has been estimated at about 10 megatons. The meteorite was mostly vaporized upon impact, leaving few remains in the crater.>>

<<Messier [crater] is very elongated (15 by 8 km), and the irregularly shaped Messier A (16 by 11 km) has long twin tails of rays that point away from Messier. Previous explanations for this crater pair have ranged from imaginative to fantastical. All were wrong.>>

Last edited by neufer on Sat Dec 02, 2017 9:20 pm, edited 1 time in total.

Thanks for addressing the impact question, especially for that correction, Neufer, on the size of the object that formed the Arizona Crater. Been there. Seen that. It's a big hole in the ground even with the erosion. Those craters on the moon with multiple kilometer diameters and walls leave you wishing to be there instead of just looking from a distance.